On the prairies, fog isn't something we have to deal with for many days each year. So when it hangs around, it's useful to know what causing it.

Fog forms when the difference between temperature and dew point is generally less than 2.5° C.

For example, on Wednesday morning, the dew point and the temperature in Regina were both 15.1° C. That created perfect conditions for the formation of fog.

When the air temperature cools to the dew point, or if the dew point rises to equal the air temperature, then dew, fog or clouds begin to form. At this point, where the dew point temperature equals the air temperature, the relative humidity is 100 per cent.

Fog is essentially a cloud that has formed at ground level.

Regina's humidity highest in July

Fog hung in downtown Regina just before sunrise, as seen from the CBC Saskatchewan station on Broad Street. (Farah Singh/CBC Saskatchewan)

Light wind, ground inversion and clear skies during the previous night are also factors that contribute to the formation of fog.

At night, the heat from the earth radiates back into space. Early in the morning, the temperature is lower near the surface than above. This is called an inversion. When it happens, the warm air doesn't rise. As the temperature falls below the so-called dew point temperature near the ground, if there is enough humidity in the air, eventually fog will form.

As soon as the temperature increases and humidity levels drop, the fog will dissipate, or 'lift'. Generally this happens due to daytime heating by the sun.